On-line mold testing takes sand quality control a step further.

Often neglected by green sand foundries, on-line mold tests more accurately reflect mold surface conditions than sand lab analysis.

Despite the introduction of many new molding methods over the last several decades, green sand remains the primary media for making metal castings. The control and maintenance of a present-day green sand system is an important and demanding job for any foundry, since the quality of the sand equals the quality of the mold, which in turn equals the quality of the casting.

The common testing done to determine sand quality falls into three categories: testing to check the consistency of prepared sand; testing to evaluate whether the sand mix has the physical and chemical properties to make a good casting; and testing to check if the molds receive the same consistent degree of compaction.

This article will address the last of these categories, particularly the on-line testing of mold compaction.

Laboratory Procedures

Tests such as moisture, compactibility, green compression, permeability, green tensile and splitting strength are important to determine if the sand leaving the sand system is consistent in these properties, and to indicate whether a good mold can be made from this sand if it is compacted correctly. Standard laboratory sand tests are run to evaluate a variety of necessary properties. Loss on ignition, Methylene Blue clay, base sand distribution and AFS clay tests tell whether the molding sand has the physical properties to withstand the hot metal or rough handling of the casting/shakeout process.

These tests, however, do not ensure that a good mold has been made. While they are valuable in their ability to indicate if the raw material is up to par, they do not say if the molds have received the correct compaction, and therefore, that a good casting will be produced.

One major problem with basing sand control on laboratory data alone is that the standard laboratory 3-ram compaction or squeeze compaction tests do not necessarily represent the same compaction the sand receives on the production molding line. A sand exhibiting acceptable properties tested under lab conditions will, if poorly or nonuniformly compacted due to process variables, produce poor molds and cause scrap.

On-Line Mold Tests

You don't pour metal against a squeezed or rammed lab specimen, you pour it into molds, and mold quality determines success. It is therefore important to complement standard sand lab analyses with on-line testing results.

Following are the several hand-held tools available to measure the compaction of green sand molds.

Mold Hardness testers - The hardness to which a sand has been compacted in the mold can be checked by one of two mold hardness testers. The 'B' scale mold hardness tester is best used on softer molds, anywhere up to a 90 mold hardness reading [ILLUSTRATION FOR FIGURE 1 OMITTED]. The instrument has a large-diameter (about 0.5 in. diameter) ball penetrator that presses into the mold surface under a spring load of 980 g. The principles is similar to the Brinell hardness test for metals.

These "B" testers give a dimensionless "B" reading from 0-100. The scale is uniform over the entire range. Penetration is displayed on a reverse dial in thousandths of an inch. The softer the mold, the greater the penetration of the ball into it. A mold that offers no resistance to penetration will have a reading of zero, while one that completely resists will read 100.

For testing denser molds, like those produced on impact or automatic mold machines, the "C" tester [ILLUSTRATION FOR FIGURE 2 OMITTED] provides more meaningful readings. The cone shaped penetrator foot is smaller (about 3/8 in. diameter) and the spring loading is 1500 g at full scale of 100. The scale readings are graduated from 65-100, spreading out the readings on the high side for greater sensitivity to harder molds.

The disadvantage of these instruments is that they can only be used on a flat surface of the mold. That surface must be more than 1/8 in. diameter to allow the tester to be correctly positioned. The reading leaves a slight dimple on the mold surface, and a slightly raised round spot on the casting.

Mold Strength Testers - Mold strength is a product of the sand's bond strength and the compaction energy used in molding. Casting defects such as mold cuts, washes and swells are products of inadequate mold surface strength.

The mold strength tester [ILLUSTRATION FOR FIGURE 3 OMITTED] is better than the "C" mold hardness tester for high-density molds. The curve of mold hardness vs. compaction flattens out above a mold hardness of 92 on the "C" scale. The mold strength tester is more sensitive in this range, and gives a reading comparable to the green compression test conducted in the lab.

This unit has a thin, round penetrator foot of 1/8 in. diameter. The penetrator is pushed into the mold until a red line on the penetrator reaches the mold surface. The maximum reading this produces on the dial is recorded as the mold strength in psi. The testers are available in two ranges: 0-30 psi and 0-50 psi. The test can be performed anywhere on the mold, regardless of surface geometry, provided the red line on the penetrator can be seen. The penetrator does form a small dimple on the mold surface and produces a small positive dimple on the casting.

A new, electronic version of the mold strength tester is also available. Some of the advantages of this unit include: a slightly more accurate reading than mechanically loaded spring penetrators; calculation by a crystal oscillator sensing element; battery powered operation; a digital readout; and it is free of indicating hands that can be damaged or bent.

Mold Permmeter Testers - The new electric mold permmeter checks surface permeability [ILLUSTRATION FOR FIGURE 4 OMITTED]. Here again, the permeability of the mold can be different than that of the 3-ram or squeezed specimen, due to sometimes dramatic differences in degree of compaction. The mold permeability correlates to the surface condition of the casting to be made in the mold, while a laboratory permeability test only determines the mold's venting ability.

The electric permmeter can be fitted with an attachment that allows mold surface permeability to be measured. The attachment is a rubber-faced contact head that is connected by rubber tubing to the permmeter. The adapter contact head must be used on a flat surface of at least 1 in. diameter on the mold. The mold permeability test leaves no mark on the mold when properly performed.

Mold Quality Indicator (MQI) - This unit [ILLUSTRATION FOR FIGURE 5 OMITTED] measures the density of a compacted sand surface by its resistance to a constant air flow under constant pressure, working off the permeability of a compacted surface. The higher the bulk density of a mold, the better the casting's surface finish will be. The battery-operated unit is digital, allowing less chance of a reading error. Like the mold permmeter, the MQI's padded contact head leaves no mark on the mold surface.

Rather than providing a measure of bond strength, as do mold hardness or mold strength tests, this unit's reading relates more directly to mold density. The tighter the mold and the greater the mold density, the higher the MQI reading. It is more sensitive to the compacted mold's permeability and, in turn, to the eventual casting surface finish. This tester has also been successfully used in nobake mold applications.

Mold Test Applications

Mold test tools provide a map of compaction during varied changes in compaction force and methods. This map can be made horizontally across the mold or on the vertical walls of the mold. The tools can gauge consistency from mold to mold by comparing the same area on each mold.

Laboratory sand analyses to establish the consistency of a sand's physical and chemical properties is the first step toward producing good castings. However, tools are needed to ensure proper process control of the molding operation, which must be able to provide the same compaction energy time after time. Through on-line mold testing, poor mold properties can be detected before substandard or scrap castings are produced.